mesa: replace _mesa_problem() with unreachable() in _mesa_light()

All drivers but the old nouveau dri driver return after this anyway.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>

src/mesa/main/light.c

@@ -186,8 +186,7 @@ _mesa_light(struct gl_context *ctx, GLuint lnum, GLenum pname, const GLfloat *pa
       light->QuadraticAttenuation = params[0];
       break;
    default:
-      _mesa_problem(ctx, "Unexpected pname in _mesa_light()");
-      return;
+      unreachable("Unexpected pname in _mesa_light()");
    }
 
    if (ctx->Driver.Lightfv)